A desirable feature in water-based pressure sensitive adhesives (PSAs) is high solids content (typically greater than about 50% solids) and low viscosity so as to allow for efficient coating. Such a PSA composition can often translate into relatively low manufacturing cost. Because less water is present with high solids content, less energy is used to evaporate the water when drying a PSA coating. The result is a faster drying rate and higher line speed for coating the PSA composition. However, a high solids content composition may translate into a high viscosity composition. If the viscosity of the PSA composition is too high, coatability and processing of the PSA composition becomes very difficult.
Various techniques have been used in emulsion polymerization to afford latex PSAs of high solids content and relatively low viscosity. For example, U.S. Pat. No. 6,048,611 (Lu et al.) describes latex PSAs, produced via emulsion polymerization, having high solids content (about 40 to 70 weight percent solid phase) that is also moisture resistant. The latex PSA uses a relatively low amount (about 2 to 5%) of a low-molecular weight hydrophobic polymer in its monomer phase and a copolymerizable surfactant, which was used to improve cohesive strength and impart moisture resistance. U.S. Pat. No. 6,225,401 (Rehmer et al.) describes a process for producing readily filterable and deodorizable aqueous dispersions of PSA having solids content of more than 65%. The dispersions are prepared using emulsion feed techniques. U.S. Pat. No. 6,706,356 (Lee) describes emulsion polymerized PSA having high solids content (at least 68%) and a viscosity in the range of 300 to 15,000 centipoise prepared using split feed, delayed monomer addition technique to create a bimodal particle size distribution.